With the need for less expensive and more specific blood typing reagents, laboratories are utilizing plant proteins for the typing of A1 and H-like red cell antigens. In order to obtain a purified well characterized type B erythroagglutinin and D-galactosyl binding protein we are studying the anti-A plus B lectin from Sophora japonica seeds. The purified lectin of 133,000 daltons exists as a tetramer of two types of monomeric polypeptide chains. One chain contains two and the other one 1/2 cystinyl residues, and one chain type possesses alanine as the N-terminal amino acid. The N-terminal amino acyl residue of the other chain has a blocked amino function. The native protein is composed of two dimeric units bound by SDS dissociable bonds. Each dimer of 68,000 dalton consists of two polypeptide chains linked by disulfide bonds. The lectin appears to exist as rapidly equilibrating interconvertible conformers of the same molecular weight. Studies with human and murine erythrocytes and lymphocytes indicate that the S. japonica lectin displays anti-I activity, as well as anti-A and B character. In addition, this lectin which lacks mitogenic and immunosuppressive properties, recognizes lymphocyte receptors distinct from those receptors available for reactivity with the mitogens, concanavalin A and M. pomifera lectin.